ASTR 1120 ASTR 1120 General Astronomy: General Astronomy: Stars & Galaxies Stars & Galaxies HOMEWORK #3 due NEXT TUE, 09/29, by 5pm Fiske planetarium: ”The Birth of Stars” by Prof. John Bally - TH 09/24-FRI 09/25, 7:30pm
Astronomer’ ’s Toolbox: s Toolbox: Astronomer What do we know how to do What do we know how to do now? now? • Measure Distance Distance: – parallax…good to nearby stars but not beyond • Measure Luminosity: Luminosity: – measure apparent brightness and distance, infer luminosity • Measure Temperature: Temperature: – Wien’s law, or, better yet, take spectra and use spectral classification. spectral classification. Next: Mass Mass
Masses are much harder than Masses are much harder than distance, luminosity, or temperature distance, luminosity, or temperature • Since we are only ever seeing a point source, it is hard to determine how much mass is contained. – If we could see another nearby object (another star maybe?) we could use the gravity between the objects as a measure of the mass.
Binary Stars to the Rescue!! Binary Stars to the Rescue!! • Types of binary star systems: – Visual Binary – Eclipsing Binary – Spectroscopic Binary About half half of all stars are in binary systems
Visual Binary Visual Binary We can directly observe the orbital motions of these stars
Eclipsing Binary Eclipsing Binary We can measure periodic eclipses
Spectroscopic Binary Spectroscopic Binary We determine the orbit by measuring Doppler shifts
Animation from http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Movies/spanim.gif
Newton’ ’s Laws of gravity s Laws of gravity Newton provide the mass provide the mass Direct mass measurements are possible only for stars in binary star systems Once we know: p = period a = average separation We can solve Newton’s equations for mass (M) Isaac Newton
Astronomer’ ’s Toolbox: s Toolbox: Astronomer What do we know how to do What do we know how to do now? now? • Measure Distance Distance: – parallax…good to nearby stars but not beyond • Measure Luminosity: Luminosity: – measure apparent brightness and distance, infer luminosity • Measure Temperature: Temperature: – Wien’s law, or, better yet, take spectra and use spectral classification. • Measure Mass: Mass: – For stars in binary For stars in binary orbits, if we can get their orbits, if we can get their orbital orbital – parameters, we can figure out their mass their mass parameters, we can figure out
Wide range of luminosities, temperatures and masses Any correlation correlation among these quantities?
The Hertzsprung Hertzsprung Russell Russell The Diagram Diagram • THIS IS AN IMPORTANT DIAGRAM TO UNDERSTAND. • Basics: – Plots Stellar Luminosity Stellar Luminosity (not apparent brightness) Vs – Temperature Temperature or Color Color or Spectral Class Spectral Class –
Study this plot! Study this plot!
Clicker Question Clicker Question Are the variables plotted here Are the variables plotted here related to each other? related to each other? A. Yes, they show a relationship B. You can’t be sure – you don’t know what they are! C. They are related to each other or else both are related to a third variable D. A or C E. None of the above
They DO show a relationship! They DO show a relationship!
� -R -R d dia iagra ra � � -R -R d dia iagra ra � Emitted power per unit area = � T 4 where � 5 ergK � 4 cm � 2 s � 1 � = 5.67 x 10 Luminosity Total luminosity from a star of radius R: 2 � T 4 L = 4 � R For the same temperature, more luminous stars have larger radii Temperature
Main sequence stars Main sequence stars • Burning hydrogen hydrogen in their cores • Stellar masses decrease downward • Temperatures are hotter hotter for more massive massive stars (more gravitational pressure � higher T, remember Equation of State) • More luminous luminous (higher T � much higher emitted power)
��� ar li life � mes alo long � e main sequenc � ��� ar li life � mes alo long � e main sequenc � Available hydrogen fuel is greater for the most massive stars… But luminosity (rate at which hydrogen is fused) is MUCH MUCH MUCH higher MUCH � More massive (more luminous) main sequence stars run out of fuel sooner Example: Most massive O star: M = 100 M Sun L = 10 6 L Sun M/L = 10 2 /10 6 = 10 -4 of the Sun Life O-Star =10 10 yrs * 10 -4 = 10 10 6 6 yrs yrs Geo Metro
Lifetimes of Main Sequence Lifetimes of Main Sequence Stars Stars • Rock-star analogy: more massive massive, hotter, more hotter luminous stars burn luminous through the available fuel faster, leading to early burnout
Lifetimes on Main Sequence (MS) Lifetimes on Main Sequence (MS) • Stars spend 90% of their lives on MS Stars spend 90% of their lives on MS • • Lifetime on MS Lifetime on MS = amount of time star = amount of time star • fuses hydrogen (gradually) in its core hydrogen (gradually) in its core fuses • For For Sun (G) Sun (G), , this is about 10 billion years this is about 10 billion years • • For For more massive stars more massive stars (OBAF), lifetime is (OBAF), lifetime is • (much) shorter (much) shorter • For For less massive stars less massive stars (KM), lifetime is (KM), lifetime is • longer longer
Clicker Question Clicker Question George and Abe are two main George and Abe are two main sequence stars; George is an M star sequence stars; George is an M star and Abe is a B star. Which is more and Abe is a B star. Which is more massive? Which is redder in color? massive? Which is redder in color? A. George is more massive and redder B. Abe is more massive and redder C. George is more massive; Abe is redder D. Abe is more massive; George is redder E. They are both main sequence, they’re the same mass and same color.
Clicker Question Clicker Question George and Abe are two main George and Abe are two main sequence stars; George is an M star sequence stars; George is an M star and Abe is a B star. Which is more and Abe is a B star. Which is more massive? Which is redder in color? massive? Which is redder in color? A. George is more massive and redder B. Abe is more massive and redder C. George is more massive; Abe is redder D. Abe is more massive; George is redder E. They are both main sequence, they’re the same mass and same color.
Main-Sequence Star Summary High Mass Mass: : High High Luminosity Short-Lived Large Radius Hot Blue Low Mass Mass: : Low Low Luminosity Long-Lived Small Radius Cool Red
What about the other objects on the H-R diagram? Luminosity As stars run out of hydrogen fuel their properties change (generally they turn into red giants- more on why next week) Temperature
• Top end of main sequence starts to “peel off” • Pleiades star cluster shown � no more O and B stars
Main- sequence turnoff point of a cluster tells us its age
Analogy: Your refrigerator Analogy: Your refrigerator Different foods have One day different shelf lives. One week Assuming you clean 3 weeks out food that goes bad promptly, the 3 months content of your 30 years refrigerator tells you how long it’s been since you went to the store
Applets Applets • "Picture" of an aging cluster • HR Diagram of an aging cluster
Clicker Question Clicker Question How do we measure the age of a stellar How do we measure the age of a stellar cluster? cluster? A. Use binary stars to measure the age of stars in the cluster. B. Use the spectral types of the most numerous stars in the cluster to infer their temperatures, and thus, the age of the cluster. C. Find stars in the instability strip and use their variability period to measure their age. D. Look for the age of stars at the main-sequence turnoff point. E. Determine if the cluster is an open cluster or globular cluster and use the average age of those types of clusters.
Clicker Question Clicker Question How do we measure the age of a stellar How do we measure the age of a stellar cluster? cluster? A. Use binary stars to measure the age of stars in the cluster. B. Use the spectral types of the most numerous stars in the cluster to infer their temperatures, and thus, the age of the cluster. C. Find stars in the instability strip and use their variability period to measure their age. D. Look for the age of stars at the main-sequence turnoff point. E. Determine if the cluster is an open cluster or globular cluster and use the average age of those types of clusters.
Clicker Question Clicker Question Main sequence A-stars have masses about 3 Main sequence A-stars have masses about 3 times that of the Sun, and luminosities about 30 times that of the Sun, and luminosities about 30 times that of the Sun. What is the age of a cluster times that of the Sun. What is the age of a cluster which has a “ “turnoff turnoff” ” at A-stars? (Remember: The at A-stars? (Remember: The which has a Sun’ ’s lifetime ~ 10 billion years) s lifetime ~ 10 billion years) Sun A) 100 thousand years B) 100 million years C) 1 billion years D) 10 billion years E) 100 billion years
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